The field of conventional mixing devices can be roughly divided into two main areas: dynamic mixers and static mixers. Dynamic or mechanical mixers rely on some type of moving part to ensure the desired and/or thorough mixing of products. Static mixers generally have no prominent moving parts and instead rely on pressure differentials within the fluids being mixed to facilitate mixing. The current disclosure is mostly directed to a static mixer but could also be applicable to dynamic mixers.
Isocyanates are molecules characterized by N═C═O functional groups. The most widely used isocyanates are aromatic compounds derived from benzene. Two polyisocyanates are widely produced commercially, namely, toluene diisocyanate (TDI) and polymeric methylenediphenyl-diisocyanate (PMDI). PMDI is a mixture of polymethylene diisocyanate and the monomeric methylenediphenyldiisocyate isomers. Ultimately, these isocyanates are reacted with polyols to form polyurethanes. Two of the major polyurethane applications are rigid foams for appliance insulation and automotive parts and flexible foams for mattresses and seating. The reaction between the amine and phosgene normally occurs at conditions where there are both mass-transfer limited or mixing controlled as well as kinetically controlled reactions. Yield losses and product quality are affected by the formation of urea and urea-derivatives in the production process. Phosgene should engulf the amine stream to minimize secondary reactions.
Mixing is important in PMDI and TDI production. The PMDI product quality and TDI yield is dependent on a multistep chemical reaction network, including a first step where two continuous streams of reactants are directed into a mixer and where, because of the residual reactivity of the compound produced in a first step of the process, secondary effects or reactions created after the primary reaction occur and ultimately reduce the quality of the product composition. For example, in the case of phosgenation chemistry, methylenedi(phenylamine) (MDA or PMDA), also referred to herein as amine, is mixed with COCl2 (phosgene) to create a mixture of Hydrochloric Acid (HCl) and Carbamyl Chloride. The chemical reaction can be depicted as follows:Amine+COCl2→HCl+Carbamyl Chloride
The Carbamyl Chloride will then decompose to isocyanate. While the production of isocyanates is desired, secondary reactions can lead to the creation of undesired by-products. Some of these secondary reactions are believed to create products such as amine hydrochloride, urea, carbodiimides, and uretonimines. Uretonimines are formed from the reaction of a carbodiimide with an isocyanate and are often called APA (Addition Product A). Since the formation of by-products, such as urea and/or uretonimines, is undesirable, the increase of the ratio of phosgene to PMDA, a dilution of PMDA in a solvent, or an improved mixing minimizes the formation of undesired by-products and fouling. Many known and unknown factors control the quality of the principal reaction.
In addition to by-product formation, improper mixing can contribute to mixer fouling. Consequently, mixer designs with improper mixing can result in lower overall yield of the desired product or can generate a product that clogs or fouls the reactor system leading to down time and/or increased maintenance costs. U.S. patent application Ser. No. 11/658,193, having a least partial common inventorship, is directed to a tapered aperture static mixer. In this application, multi-tee mixers include a tee-pipe junction and a straight pipe section with nozzles and blind flanges for the rapid initiation of the chemical reaction. The junction at these prior art multi-tee static mixers includes a mixing chamber having separate inlets for at least two components and an outlet. The inlet for one of the components is defined along a longitudinal axis of the multi-tee mixer and the inlet for the other component(s) is formed as a plurality of nozzles or jets disposed around the circumference of the mixing chamber and oriented normal to the longitudinal axis of the multi-tee mixer.
In another reference, U.S. patent application Ser. No. 12/725,262 filed on Mar. 16, 2010, also by at least a partial common inventorship, the length of the principal conduit downstream of a mixing area is minimized to limit improper mixing and the creation of by-products. In yet another reference, U.S. patent application Ser. No. 12/725,266 filed on Mar. 16, 2010, also by at least a partial common inventorship, improper mixing is reduced via the introduction of a guide element into the main conduit of a static reactor to create uniform flow of incoming phosgene into a ring of limited thickness so circumferential nozzles can spread amine with a greater contact area of phosgene. While these references teach improved mixing and are incorporated fully herein by reference, further improvements are desirable to enhance mixing of component materials.
FIG. 1 shows phosgene concentrations within a fluid receiving chamber within a first cylindrical conduit where a flow of phosgene from the left to the right of the figure evolves. Amine is jetted into an incoming flow of a first component. As the amine traverses and reacts with the phosgene, principal and secondary reactions occur. A circle disposed at the distance L illustrates a region on the downstream side of the amine jet where the phosgene concentration is relatively low (near zero). The associated temperature map shown as FIG. 2 illustrates the distribution of temperature within the mixture due to the overall exothermic chemical reactions. Again, the circle disposed at distance L1 downstream from the nozzle is shown to be farther than the distance L at FIG. 1, and the local temperature is increased promoting the formation of secondary reactions and associated by-products. While one location of relatively low phosgene concentration (L) and of increased local temperature (L1) is shown at FIGS. 1-2, one of ordinary skill in the art will recognize that these values are only indicative of the general effect and may change based on a plurality of factors including but not limited to fluid viscosity, fluid velocity, temperature, reactant concentrations, pressure, etc.
What is needed is a static reactive jet mixer capable of limiting peaks of concentration and temperature within the principal stream of phosgene and amine during the mixing process thus limiting the production of urea or other undesired by-products in the static mixer.